AN ASSESSMENT OF THE POTENTIAL IMPACT OF ALIEN INVASIVE VEGETATION ON THE GEOMORPHOLOGY OF RIVER CHANNELS IN SOUTH AFRICA

Summary

Invasion of the riparian zone by alien vegetation is recognised as a serious problem in many areas of South Africa. Vegetation is a dynamic component of river channels. It is an important control variable affecting channel form whereas the flow and sediment regime influences vegetation growth. Wherever alien vegetation invades the riparian zone it can be expected that there will be some impact on the physical structure of the riparian habitat. This paper reviews the effect of riparian vegetation on channel processes and channel form and discusses the implications of the invasion of riparian zones by alien vegetation. Woody species in particular are seen as having a significant potential for inducing channel modification, whilst their removal could lead to significant channel instability and mobilisation of sediment. The need for further research into the impact of alien vegetation on the geomorphology of South African river channels is stressed.     

Ecological change in the lower Omo Valley around 2.8 Ma

Late Pliocene climate changes have long been implicated in environmental changes and mammalian evolution in Africa, but high-resolution examinations of the fossil and climatic records have been hampered by poor sampling. By using fossils from the well-dated Shungura Formation (lower Omo Valley, northern Turkana Basin, southern Ethiopia), we investigate palaeodietary changes in one bovid and in one suid lineage from 3 to 2 Ma using stable isotope analysis of tooth enamel. Results show unexpectedly large increases in C₄ dietary intake around 2.8 Ma in both the bovid and suid, and possibly in a previously reported hippopotamid species. Enamel δ¹³C values after 2.8 Ma in the bovid (Tragelaphus nakuae) are higher than recorded for any living tragelaphin, and are not expected given its conservative dental morphology. A shift towards increased C₄ feeding at 2.8 Ma in the suid (Kolpochoerus limnetes) appears similarly decoupled from a well-documented record of dental evolution indicating gradual and progressive dietary change. The fact that two, perhaps three, disparate Pliocene herbivore lineages exhibit similar, and contemporaneous changes in dietary behaviour suggests a common environmental driver. Local and regional pollen, palaeosol and faunal records indicate increased aridity but no corresponding large and rapid expansion of grasslands in the Turkana Basin at 2.8 Ma. Our results provide new evidence supporting ecological change in the eastern African record around 2.8 Ma, but raise questions about the resolution at which different ecological proxies may be comparable, the correlation of vegetation and faunal change, and the interpretation of low δ¹³C values in the African Pliocene.     

A late Holocene climate record from a stalagmite, Cold Air Cave, Northern Province, South Africa

Stable isotope analyses of a uranium-series-dated stalagmite from South Africa provide a record of climate changes for the periods 4400–4000 years and 800 years ago to recent, interrupted by a prolonged growth hiatus. Generally enriched stable oxygen isotope values, interpreted here to indicate more humid conditions, occurred around 800 years ago. Subsequently a marked depletion in oxygen and carbon isotope values occurred about 600 years ago, reflecting, we believe, shifts toward drier, cooler conditions as the regional indication of the Little Ice Age. This period with depleted, yet oscillating isotope values, is replaced by a period with enriched isotopes until recent times. The record is notable for sharp shifts in isotopic values, on the scale of decades, which reflect rapid oscillations in local climate conditions.     

Food source of riparian spiders analyzed by using stable isotope ratios

We analyzed the food source of riparian spiders in a middle reach of the Chikuma River, Japan, by using stable isotope ratios of carbon and nitrogen. The carbon and nitrogen isotope ratios of attached algae were higher than those of terrestrial plants, reflecting a large carbon isotope fractionation in terrestrial plants and a difference in nitrogen sources. The carbon isotope ratios of terrestrial insects were similar to those of the terrestrial plants, and the ratios of aquatic insects were scattered between those of the terrestrial plants and the attached algae. The carbon and nitrogen isotope ratios of spiders were intermediate between those of the terrestrial and aquatic insects. The two-source mixing model using the carbon isotope ratio showed that the web-building spiders utilized both the terrestrial and aquatic insects, with large contribution by the aquatic insects (54% on average with a maximum of 92% among spiders taxa collected in each zone), in the riparian area in a middle reach of the Chikuma River. The large contribution of the aquatic insects was often observed for the spiders collected near river channel (<5m) and for the horizontal web-building spiders collected across the riparian area. The relative contribution of the aquatic insects might be related with food availability (distance from river channel) and spiders food preference reflected in their web types (horizontal vs. vertical). Our results showed that organic materials produced in the river channel, in the riparian area, and in the terrestrial area surrounding the riparian area were mixed at the carnivorous trophic level of riparian spiders.     

Nutrient Vectors and Riparian Processing: A Review with Special Reference to African Semiarid Savanna Ecosystems

Abstract This review article describes vectors for nitrogen and phosphorus delivery to riparian zones in semiarid African savannas, the processing of nutrients in the riparian zone and the effect of disturbance on these processes. Semiarid savannas exhibit sharp seasonality, complex hillslope hydrology and high spatial heterogeneity, all of which ultimately impact nutrient fluxes between riparian, upland and aquatic environments. Our review shows that strong environmental drivers such as fire and herbivory enhance nitrogen, phosphorus and sediment transport to lower slope positions by shaping vegetative patterns. These vectors differ significantly from other arid and semiarid ecosystems, and from mesic ecosystems where the impact of fire and herbivory are less pronounced and less predictable. Also unique is the presence of sodic soils in certain hillslopes, which substantially alters hydrological flowpaths and may act as a trap where nitrogen is immobilized while sediment and phosphorus transport is enhanced. Nutrients and sediments are also deposited in the riparian zone during seasonal, intermittent floods while, during the dry season, subsurface movement of water from the stream into riparian soils and vegetation further enrich riparian zones with nutrients. As is found in mesic ecosystems, nutrients are immobilized in semiarid riparian corridors through microbial and plant uptake, whereas dissimilatory processes such as denitrification may be important where labile nitrogen and carbon are in adequate supply and physical conditions are suitable—such as in seeps, wallows created by animals, ephemeral wetlands and stream edges. Interaction between temporal hydrologic connectivity and spatial heterogeneity are disrupted by disturbances such as large floods and extended droughts, which may convert certain riparian patches from sinks to sources for nitrogen and phosphorus. In the face of increasing anthropogenic pressure, the scientific challenges are to provide a basic understanding of riparian biogeochemistry in semiarid African savannas to adequately address the temporal and spatial impact of disturbances, and to apply this knowledge to better regional land and water management. An integrated, multidisciplinary approach applied in protected as well as human-disturbed ecosystems in southern Africa is essential for underpinning a strong environmental basis for sustainable human-related expansion.     

The Evolution of Conservation Management Philosophy: Science, Environmental Change and Social Adjustments in Kruger National Park

In this ‘perspectives’ article, we share experiences gained from the century-old Kruger National Park (KNP) in South Africa to illustrate the dynamic complexity of biophysical and socio-political systems, the interactions that occur between them, and the consequences for ecosystem-scale functions and resources and for their management. As in KNP, the social-ecological milieu surrounding many national parks and protected areas is changing rapidly. There will be significant managerial adjustments as human populations grow and the needs for resources accelerate. The changes, driven largely by global-scale environmental shifts as well as by new knowledge, are intimately intertwined with evolving societal perceptions, values, and expectations. Many KNP resource-related issues of the past century originated more internally and were largely environmental, whereas the emerging issues are more external and largely social. Here, we illustrate how interrelated scientific and managerial advances in integrating biophysical and social systems are acting to conserve and rehabilitate resources within KNP, and to aid in their conservation. Where appropriate, we relate these advances to similar examples in the region or other protected areas in the world. Strategies to address emerging issues are identified and discussed—and their combined effects on resource conservation and management are evaluated. In our experience the approach to conservation within KNP has been successful, despite well-intended but damaging management actions in the past. We believe that the perceived success stems from a willingness to continually incorporate new knowledge into management, to foster close working and personal associations among scientists, managers, and rangers, to acquire an intimate knowledge and understanding of the social-ecological system by the administrators as well as by the staff, and to be actively ‘forward’ thinking in an increasingly complex and uncertain world. We accept that many decisions taken today will be challenged by future managers and scientists, and we expect that some will be found wanting as emerging knowledge and continued learning shape future decisions. Further, evolving political, social, and environmental contexts may mean that protected areas will need to be managed in different ways. Therefore, we emphasize the importance of minimizing the permanency and impact of decisions so that today’s actions do not compromise future decisions when meaningful changes need to be made.

华南主要野生蔬菜的脂肪酸成分分析

本实验以华南主要野生蔬菜守宫木、土人参、一点红、白仔菜、紫背菜、鳄嘴花、藤三七、塘葛菜为材料,并以华南特产蔬菜菜心为对照,对8种野生蔬菜的脂肪酸成分进行了分析.结果表明:8种野菜共检出十四酸、软脂酸、棕榈油酸、硬脂酸、油酸、亚油酸、亚麻酸、二十二酸和二十四酸共9种脂肪酸,但不同野菜之间的脂肪酸组成与含量差异极大.8种野菜的饱和脂肪酸的总量都高于菜心,饱和脂肪酸种类最多的是藤三七,含4种.不饱和脂肪酸的变化与饱和脂肪酸相反,8种野菜都低于菜心,但其油酸、亚油酸远高过菜心.菜心富含亚麻酸,但不含亚油酸.可见8种野菜油营养价值较高.     

稳定同位素技术在植物水分利用研究中的应用

近20a稳定同位素技术在植物生态学研究中的应用得到了长足发展,使得对植物与水分关系也有了更深一步的了解。介绍稳定同位素性碳、氢、氧同位素在研究植物水分关系中的应用及进展,以期能为国内植物水分利用研究提供参考。由于植物根系从土壤中吸收水分时并不发生同位素分馏,对木质部水分同位素分析有助于对植物利用水分来源,生态系统中植物对水分的竞争和利用策略的研究,更好地了解生态系统结构与功能。稳定碳同位素作为植物水分利用效率的一个间接指标,在不同水分梯度环境中,及植物不同代谢产物与水分关系中有着广泛的应用。同位素在土壤-植被-大气连续体水分中的应用,有助于了解生态系统的水分平衡。随着稳定同位素方法的使用,植物与水分关系的研究将取得更大的进展。     

海洋生态系统稳定同位素基线的选取

稳定同位素技术在海洋食物网研究中得到了广泛的应用,但在分析海洋生物食性和营养级时,需要选择某种生物或物质的稳定同位素值作为基线.本文综述了河口和海湾、浅海、大洋及深海4类典型海洋生态系统稳定同位素基线的选取,分析了影响稳定同位素基线选择的因素,以及特定化合物稳定同位素在消除基线时空异质性中的潜在价值,并对目前存在的问题以及今后的研究方向进行了展望,以期为国内学者进一步深入开展海洋生态系统的稳定同位素生态学研究提供有益参考.